1. Technical Field
The present invention relates to control devices for vehicle drive apparatuses, and more particularly, to a technology of appropriately limiting a rotation speed of an electric motor in a hybrid vehicle drive apparatus having a differential mechanism operative to perform a differential action.
2. Background Art
An attempt has heretofore been made to provide a four-wheel drive vehicle in which front wheels and an electric power generator are driven with an engine and rear wheels are driven with an electric motor directly via a clutch engaging device upon receipt of electric power supplied from the electric power generator. There has heretofore been known a drive control device for such a four-wheel drive vehicle in which when disengaging the clutch engaging device to switch a four-wheel drive state to a front-wheel drive state, both operations are executed to disengage the clutch engaging element and interrupting the supply of electric power to the electric motor while executing a control to cut off a field current of such an electric motor. For instance, Patent Publication 1 (Japanese Patent Application Publication No. 2004-266958A) discloses a drive control device representing the drive control device set forth above. There has heretofore been also known a drive control device disclosed in Patent Publication 2 (Japanese Patent Application Publication No. 2005-172044A).
A hybrid vehicle drive apparatus, to which the present invention is related, includes an electrically controlled differential portion having a differential mechanism including a first rotary element connected to an engine in a drive-force transmissive state, a second rotary element connected to a first electric motor in a drive-force transmissive state, and a third rotary element connected to a power transmitting path leading to drive wheels, a second electric motor connected to the power transmitting path, and an automatic shifting portion forming a part of the power transmitting path to function as an automatic power transmission. With such a hybrid vehicle drive apparatus, if a vehicle occupant operates, for instance, a shift lever to cause a disconnection in the power transmitting path from the second electric motor to the drive wheels, the differential mechanism performs a differential action and, hence, it becomes complicated to control a rotation speed of the second electric motor. This results in a probability with the second electric motor rotating at a high speed exceeding a permit rotation speed. With the second electric motor rotating at such a high speed, there is the possibility of causing degradation in durability of the second electric motor.
To address such an issue, even if the invention disclosed in Patent Publication 1 is applied so as to interrupt the supply of electric power to the second electric motor in synchronism with the disconnecting operation of the power transmitting path, the rotation speed of the second electric motor remains unchanged under the complicated control. This is because the rotation speed of the second electric motor depends on a rotation speed of the engine and a rotation speed of the first electric motor due to the differential action mentioned above. Thus, under a circumstance where the second electric motor is applied with high torque from the engine and rotates at a speed in the vicinity of the permit rotation speed, difficulty has been encountered in preventing the second electric motor from rotating at a high speed.